Fix invalid handle scope error when Dart_IsClosure is used.

runtime/vm/object.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/object.h"
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "vm/assembler.h"
 #include "vm/cpu.h"
@@ skipping 13171 matching lines @@
     StorePointer(NativeFieldsAddr(), native_fields.raw());
   }
   for (uint16_t i = 0; i < num_native_fields; i++) {
     intptr_t byte_offset = i * sizeof(intptr_t);
     TypedData::Cast(native_fields).SetIntPtr(byte_offset, field_values[i]);
   }
 }
 
 
 bool Instance::IsClosure() const {
-  const Class& cls = Class::Handle(clazz());
-  return cls.IsSignatureClass();
+  return Class::IsSignatureClass(clazz());
 }
 
 
 bool Instance::IsCallable(Function* function, Context* context) const {
   Class& cls = Class::Handle(clazz());
   if (cls.IsSignatureClass()) {
     if (function != NULL) {
       *function = Closure::function(*this);
     }
     if (context != NULL) {
@@ skipping 5961 matching lines @@
   return tag_label.ToCString();
 }
 
 
 void UserTag::PrintJSONImpl(JSONStream* stream, bool ref) const {
   Instance::PrintJSONImpl(stream, ref);
 }
 
 
 }  // namespace dart